Teen pop idol David Cassidy performs at Madison Square Garden,...

Teen pop idol David Cassidy performs at Madison Square Garden, on March 11, 1972. Credit: AP Photo

Poor little Petra, just 13 and growing up in South Wales, is one of thousands of girls smitten with David Cassidy. The year is 1974, and Cassidy is the creation of a brilliant marketing campaign that preys on the hearts of teenage girls.

As you may or may not remember, depending on which planet you were born on, Cassidy was the star of the TV series "The Partridge Family" (more recently, he was a contestant on "The Celebrity Apprentice"). He branched off into his own career as a bubblegum rock icon. Cassidy's career fizzled and died in a series of media scandals, including a spread of him naked in Rolling Stone (photographed by Annie Leibovitz) and admitted drug and alcohol problems. At a Cassidy concert in London's White City Stadium, a 14-year-old girl was crushed to death and nearly 1,000 fans in the crowd of 35,000 were injured.

When the novel "I Think I Love You" opens, Petra and her friends are blissfully ignorant of these eventualities. They are too busy studying up on Cassidy -- reading every little scrap, every magazine article -- collecting posters and other paraphernalia. Petra aces all the Cassidy quizzes and enters a mega-contest. First prize -- a trip to California to meet him in person.

As for the boys Petra's own age -- hopeless, useless. And the girls -- mean as ever. "You chose the kind of friends you wanted because you hoped you could be like them and not like you. To improve your image, you made yourself more stupid and less kind." Who wouldn't try to escape, if only in imagination? Petra writes letters to her true love and, lo!, she receives replies.

Little does she know that these replies are written by a would-be student of literature. Yes, Bill is hired to write replies to the thousands of girls who bare their souls each day. It pays the rent.

In this delightful, giddy novel, there can be no doubt that their lives will one day collide.

They meet twice. Once in 1974, when Bill is sent to the concert at White City Stadium, and again in 1998. Back in 1974, Petra was too disillusioned by the White City debacle to collect her prize for the great Cassidy quiz. In 1998, she makes a drunken call to the magazine, demanding her trip to America to meet her childhood heartthrob.

What possesses Petra, now 38, to make that call? Well, her mother's death; her husband's announcement that he wants a divorce; her own teenager, Molly, who has begun the excruciating rejection process known as individuation. The magazine's editors -- including Bill, who has graduated to editorial director -- sense a golden opportunity. They will fly the middle-aged woman to meet the aging superstar in Vegas. Imagine Petra's surprise when she learns that Bill was the letter writer, the literary hand of David Cassidy.

It's a fantastic, unbelievable plot, spun from the impossibility of celebrity adoration and the gorgeous naivete of young love. Allison Pearson, whose novel "I Don't Know How She Does It" sold more than 500,000 copies in 31 languages, is the master of the balancing act.

Somewhere in all this frippery, she finds those universal chords: what it means, as a girl, to search the world for clues upon which nothing less than one's survival depends -- what to wear, what to want, whom to trust. And the desire not to be one's mother at all costs, the raw energy that makes a girl want to escape a world in which she cannot possibly ever be appreciated or fully realized.

The novel was born out of Pearson's experience, seven years ago, interviewing Cassidy for the Daily Telegraph Saturday magazine. Like Petra, she was once a crazy fan. "I felt like a time traveler," she writes in her afterword. "If David was still 24 in my heart, how old did that make me?" In that interview, Pearson asks Cassidy the question that has been nagging at her all those years. "David Cassidy, was your favorite color ever brown?"

"Brown?" Cassidy replies. "Never. No."

"For 18 months I wore nothing but brown because I read in a magazine it was your favorite color."

"Allison," Cassidy replies, "it was all made up!"

I THINK I LOVE YOU, by Allison Pearson. Alfred A. Knopf, 357 pp., $24.95.

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